Writing into Uninitialized Buffers in Rust

https://blog.sunfishcode.online/writingintouninitializedbuffersinrust/

129•luu•1mo ago

Comments

90s_dev•1mo ago

Write into uninit'd buffers was one of the pain points of Rust for the creator of the new open source "edit" program for Windows[1]. I wonder what he thinks of this article.

> Another thing is the difficulty of using uninitialized data in Rust. I do understand that this involves an attribute in clang which can then perform quite drastic optimizations based on it, but this makes my life as a programmer kind of difficult at times. When it comes to `MaybeUninit`, or the previous `mem::uninit()`, I feel like the complexity of compiler engineering is leaking into the programming language itself and I'd like to be shielded from that if possible. At the end of the day, what I'd love to do is declare an array in Rust, assign it no value, `read()` into it, and magically reading from said array is safe. That's roughly how it works in C, and I know that it's also UB there if you do it wrong, but one thing is different: It doesn't really ever occupy my mind as a problem. In Rust it does. [https://news.ycombinator.com/item?id=44036021]

electrograv•1mo ago

> That's roughly how it works in C, and I know that it's also UB there if you do it wrong, but one thing is different: It doesn't really ever occupy my mind as a problem. In Rust it does.

UB doesn’t occupy the author’s mind when writing C, when it really should. This kind of lazy attitude to memory safety is precisely why so much C code is notoriously riddled with memory bugs and security vulnerabilities.

mk12•1mo ago

There is an important difference for this case though. It C it’s fine to have pointers into uninitialized memory as you as you don’t read them until after initializing. You can write through those pointers the same way you always do. In Rust it’s UB as soon as you “produce” an invalid value, which includes references to uninitialized memory. Everything uses references in Rust but when dealing with uninitialized memory you have to scrupulously avoid them, and instead write through raw pointers. This means you can’t reuse any code that writes through &mut. Also, the rules change over time. At one point I had unsafe code that had a Vec of uninitialized elements, which was ok because I never produced a reference to any element until after I had written them (through raw pointers). But they later changed the Vec docs to say that’s UB, I guess because they want to reserve the right to use references even if you never call a method that returns a reference.

Arnavion•1mo ago

This stopped being much of a problem when MaybeUninit was stabilized. Now you can stick to using &MaybeUninit<T> / &mut MaybeUninit<T> instead of needing to juggle *T / *mut T and carefully track converting that to &T / &mut T only when it's known to be initialized, and you can't accidentally use a MaybeUninit<T> where you meant to use a T because the types are different.

It's not as painless as it could be though, because many of the MaybeUninit<T> -> T conversion fns are unstable. Eg the code in TFA needs `&mut [MaybeUninit<T>] -> &mut [T]` but `[T]::assume_init_mut()` is unstable. But reimplementing them is just a matter of copying the libstd impl, that in turn is usually just a straightforward reinterpret-cast one-liner.

nemothekid•1mo ago

Bizarre. I think I've been writing broken Rust code for a couple years. If I understand you correctly something like:

    let mut data = Vec::with_capacity(sz);
    unsafe { data.set_len(sz) };
    buf.copy_to_slice(data.as_mut_slice());

is UB?

vgatherps•1mo ago

Yes, this is the case that I ran into as well. You have to zero memory before reading and/or have some crazy combination of tracking what’s uninitialized capacity or initialized len, I think the rust stdlib write trait for &mut Vec got butchered over this concern.

It’s strictly more complicated and slower than the obvious thing to do and only exists to satisfy the abstract machine.

Arnavion•1mo ago

No. The correct way to write that code is to use .spare_capacity_mut() to get a &mut [MaybeUninit<T>], then write your Ts into that using .write_copy_of_slice(), then .set_len(). And that will not be any slower (though obviously more complicated) than the original incorrect code.

vgatherps•1mo ago

Oh this is very nice, I think it was stabilized since I wrote said code.

nemothekid•1mo ago

write_copy_of_slice doesn't look to be stable. I'll mess around with godbolt, but my hope that whatever incantation is used compiles down to a memcpy

Arnavion•1mo ago

As I wrote in https://news.ycombinator.com/item?id=44048391 , you have to get used to copying the libstd impl when working with MaybeUninit. For my code I put a "TODO(rustup)" comment on such copies, to remind myself to revisit them every time I update the Rust version in toolchain.toml

nemothekid•1mo ago

In other words the """safe""" stable code looks like this:

    let mut data = Vec::with_capacity(sz);
    let mut dst_uninit = data.spare_capacity_mut();
    let uninit_src: &[MaybeUninit<T>] = unsafe { transmute(buf) };
    dst_uninit.copy_from_slice(uninit_src);
    unsafe { data.set_len(sz) };

Arnavion•1mo ago

That's correct.

eptcyka•1mo ago

Valgrind it :)

vlovich123•1mo ago

Valgrind doesn’t tell you about UB, just if the code did something incorrect with memory and that depends on what the optimizer did if you did write UB code. You’ll need Miri to tell you if this kind of code is triggering UB which works by evaluating and analyzing the mid level of compiler output to check if Rust rules about safety are followed.

eptcyka•1mo ago

Reading from uninitialised memory is a fault that valgrind will detect.

vlovich123•1mo ago

But that’s precisely NOT the problem that exists in OPs code. It’s a problem Valgrind will detect if and only if the optimizer does something weird to exploit the UB in the code which may or may not happen AND doesn’t even necessarily happen on that line of code which will leave you scratching your head.

UB is weird and valgrind is not a tool for detecting UB. For that you want Miri or UBSAN. Valgrind’s equivalent is ASAN and MSAN which catch UB issues incidentally in some rare cases and not necessarily where the UB actually happened.

ironhaven•1mo ago

That's right. Line 3 is undefined behaviour because you are creating mutable references to the uninit spare capacity of the vec. copy_to_slice only works with writing to initialized slices. The proper way for you example to mess with the uninitialized memory on a vec would be only use raw pointers or calling the newly added Vec::spare_capacity_mut function on the vec that returns a slice of MaybeUninit

NobodyNada•1mo ago

It's an open question whether creating a reference to an uninitialized value is instant UB, or only UB if that reference is misused (e.g. if copy_to_slice reads an uninitialized byte). The specific discussion is whether the language requires "recursive validity for references", which would mean constructing a reference to an invalid value is "language UB" (your program is not well specified and the compiler is allowed to "miscompile" it) rather than "library UB" (your program is well-specified, but functions you call might not expect an uninitialized buffer and trigger language UB). See the discussion here: https://github.com/rust-lang/unsafe-code-guidelines/issues/3...

Currently, the team is leaning in the direction of not requiring recursive validity for references. This would mean your code is not language UB as long as you can assume `set_len` and `copy_to_slice` never read from 'data`. However, it's still considered library UB, as this assumption is not documented or specified anywhere and is not guaranteed -- changes to safe code in your program or in the standard library can turn this into language UB, so by doing something like this you're writing fragile code that gives up a lot of Rust's safety by design.

bombela•1mo ago

Why not simply:

    let mut data = Vec::with_capacity(sz);
    data.extend(&buf[..sz]);

Vec::extend extends a container from an iterable. A Vec/slice is iterable.

And from the doc:

> This implementation is specialized for slice iterators, where it uses copy_from_slice to append the entire slice at once.

Of course this trivial example could also be written as:

    let mut data = buf.clone();

uecker•1mo ago

It is also not UB to read uninitialized values through a pointer in C for types that do not have non-value representations.

codeflo•1mo ago

I don’t get the difference. In both C and Rust you can have pointers to uninitialized memory. In both languages, you can’t use them except in very specific circumstances (which are AFAIK identical).

There are two actual differences in this regard: C pointers are more ergonomic than Rust pointers. And Rust has an additional feature called references, which enable a lot more aggressive compiler optimizations, but which have the restriction that you can’t have a reference to uninitialized memory.

mk12•1mo ago

I agree with you. My point is that the additional feature (references) creates a new potential for UB that doesn’t exist in C, and that justifies the “doesn't really ever occupy my mind as a problem” statement being criticized upthread. You can’t compare C to Rust-without-references because no one writes Rust that way. It’s not like C++-without-exceptions which is a legitimate subset that people use.

o11c•1mo ago

The reason this particular UB doesn't need mindspace for C programmers is because it's not even meaningful to do anything with the parts of the buffer beyond the written length.

Most other UBs related to datums that you think you can do something with.

usefulcat•1mo ago

I suspect that the main reason it doesn't really occupy the author's mind is that even though it's possible to misuse read(), it's really not that hard to actually use it safely.

It sounds like the more difficult problem here has to do with explaining to the compiler that read() is not being used unsafely.

lhecker•1mo ago

What I meant is that if I write a UTF8 --> UTF16 conversion function for my editor in C I can write

  size_t convert(state_t* state, const void* inp, void* out)

This function now works with both initialized and uninitialized data in practice. It also is transparent over whether the output buffer is an `u8` (a byte buffer to write it out into a `File`) or `u16` (a buffer for then using the UTF16). I've never had to think about whether this doesn't work (in this particular context; let's ignore any alignment concerns for writes into `out` in this example) and I don't recall running into any issues writing such code in a long long time.

If I write the equivalent code in Rust I may write

  fn convert(&mut self, inp: &[u8], out: &mut [MaybeUninit<u8>]) -> usize

The problem is now obvious to me, but at least my intention is clear: "Come here! Give me your uninitialized arrays! I don't care!". But this is not the end of the problem, because writing this code is theoretically unsafe. If you have a `[u8]` slice for `out` you have to convert it to `[MaybeUninit<u8>]`, but then the function could theoretically write uninitialized data and that's UB isn't it? So now I have to think about this problem and write this instead:

  fn convert(&mut self, inp: &[u8], out: &mut [u8]) -> usize

...and that will also be unsafe, because now I have to convert my actual `[MaybeUninit<u8>]` buffer (for file writes) to `[u8]` for calls to this API.

Long story short, this is a problem that occupies my mind when writing in Rust, but not in C. That doesn't mean that C's many unsafeties don't worry me, it just means that this _particular_ problem type described above doesn't come up as an issue in C code that I write.

Edit: Also, what usefulcat said.

ninkendo•1mo ago

Why wouldn’t you accept a &mut [MaybeUninit<T>] and return a &mut [u8], hiding the unsafe bits that transmute the underlying reference?

Something like:

  fn convert<'i, 'o>(inp: &'i [u8], buf: &'o mut MaybeUninit<u8>) -> &'o mut [u8]

(Honest question, actually… because the above may be impossible to write and I’m on my phone and can’t try it.)

Edit: it works: https://play.rust-lang.org/?version=stable&mode=debug&editio...

lhecker•1mo ago

That's a fair workaround for my specific example. But I believe it's possible to contrive a different example where such a solution would not be possible. Put differently, I only tried to convey the overall idea of what I think is a shortcoming in Rust at the moment.

Edit: Also, I believe your code would fail my second section, as the `convert` function would have difficulty accepting a `[u8]` slice. Converting `[u8]` to `[MaybeUninit<u8>]` is not safe per se.

ninkendo•1mo ago

Yeah, you’d need to do something like accept an enum that is either &mut [u8] or &mut [MaybeUninit<u8>], and make a couple of impl From<>’s so callers can .into() whatever they want to pass…

But I don’t think this is really a shortcoming, so much as a simple consequence of strong typing. If you want take “whatever” as a parameter, you have to spell out the types that satisfy it, whether it’s via a trait, or an enum with specific variants, etc. You don’t get to just cast things to void and hope for the best, and still call the result safe.

ii41•1mo ago

I think this solves his problem. He said he wants a read function that turns the unsafe buffer into a safe buffer, and this API does that.

IIRC it's not that hard to convince the compiler to give you a safe buffer from a MaybeUninit. However, this type has really lengthy docs and makes you question everything you do with it. Thinking through all this is painful but it's not like you don't have to it with C.

jcranmer•1mo ago

The basic problem with uninitialized buffers is that they effectively require write-only references to exist, and Rust's type system doesn't have (and doesn't easily support) write-only references, only read-only and read-write. MaybeUninit is a partial solution to the problem, but since it's a library solution and not a language solution, it suffers from a lack of integration with the language, e.g., getting MaybeUninit fields from a MaybeUninit struct is challenging.

And the most aggravating part of all of this is that the most common use case for uninitialized memory (the scenario being talked about both in the article here and the discussion you quote) is actually pretty easy to have a reasonable, safe abstraction for, so the fact that the current options requires both use of unsafe code and also potentially faulty duplication of value calculations doesn't make for a fun experience. (Also, the I/O traits predate MaybeUninit, which means the most common place to want to work with uninitialized memory is one where you can't do it properly.)

90s_dev•1mo ago

Then is the solution to have write-only be the default for muts, so that they start out write-only at least, but can also be made to be read-write either (a) in given circumstances of creation, or (b) after certain operations on them, or (c) if created by certain APIs?

lhecker•1mo ago

Abstracting away the `assume_init` is a great idea! I think I could use something like that for the editor. The only concern I have is that the `read` function is templated on the parameter type. I'd ideally _really_ prefer it if I didn't need two copies of the same function to switch over `[u8]` and `[MaybeUninit<u8>]` due to different return types. [^1] I guess the approach could be tuned to avoid this?

Personally, I also like the simpler approach overall, compared to the `BorrowedBuf` trait, for the same reasons outlined in the article.

While this possibly solves parts of pain points that I had, what I meant to write is that in an ideal world I could write Rust while mostly not thinking about this issue much, if at all. Even with this approach, I'd still need to decide whether my API needs to take a `[u8]` or a `Buffer`, just in the mere off-chance that a caller may want to pass an uninitialized array further up in the call chain. This then requires making the call path generic for the buffer parameter which may end up duplicating any of the functions along the path, even though that's not really my intention by marking it as `Buffer`.

I think if there was a way to modify Rust so we can boldly state in writing "You may cast a `[MaybeUninit<T>]` into a `[T]` and pass it into a call _if_ you're absolutely certain that nothing reads from the slice", it would already go a long way. It may not make this more comfortable yet, but it would definitely take off a large part of my worries when writing such unsafe casts. That's basically what I meant with "occupy my mind": It's not that I wouldn't think about it at all, rather it just wouldn't be a larger concern for me anymore, for code where I know for sure that this requirement is fulfilled (i.e. similar to how I know it when writing equivalent C code).

Edit: jcranmer's suggestion of write-only references would solve this, I think? https://news.ycombinator.com/item?id=44048450

[^1]: This is of course not a problem for a simple `read` syscall, but may be an issue for more complex functions, e.g. the UTF8 <> UTF16 converter API I suggested elsewhere in this thread, particularly if it's accelerated, the way simdutf is.

vgatherps•1mo ago

I wish that there was a useful “freeze” intrinsic exposed, even if only for primitive types and not for generic user types, where the values of the frozen region become unspecified instead of undefined. I believe llvm has one now?

Iirc the work on safe transmute also involves a sort of “any bit pattern” trait?

I’ve also dealt with pain implementing similar interfaces in Rust, and it really feels like you end up jumping through a ton of hoops (and in some of my cases, hurting performance) all to satisfy the abstract machine, at no benefit to programmer or application. It’s really a case where the abstract machine cart is leading the horse

CJefferson•1mo ago

I agree, I'd go further and say I wonder why primitive types aren't "frozen" by default.

I totally understand not wanting to promise things get zeroed, but I don't really understand why full UB, instead of just "they have whatever value is initially in memory / the register / the compiler chose" is so much better.

Has anyone ever done a performance comparison between UB and freezing I wonder? I can't find one.

wrs•1mo ago

That assumes the compiler reserves one continuous place for the value, which isn’t always true (hardly ever true in the case of registers). If the compiler is required to make all code paths result in the same uninitialized value, that can limit code generation options, which might reduce performance (and performance is the whole reason to use uninitialized values!).

Also, an uninitialized value might be in a memory page that gets reclaimed and then mapped in again, in which case (because it hasn’t been written to) the OS doesn’t guarantee it will have the same value the second time. There was recently a bug discovered in one of the few algorithms that uses uninitialized values, because of this effect.

CJefferson•1mo ago

But, I wonder how much it would reduce performance, if we only have to pick a value the first time the memory is read?

I would imagine there isn't that many cases where we are reading uninitalised memory and counting on that reading not saving a value. It would happen when reading in 8-byte blocks for alignment, but does it happen that much elsewhere?

ablob•1mo ago

if you pick a value you have to store it, and if you have to store it it might spill into memory when register allocation fails. Moving from register-only to stack/heap usage easily slows down your program by an order of magnitude or two. If this is in a hot path, which I'd argue it is since using uninitialized values seems senseless otherwise, it might have a big impact.

The only way to really know is to test this. Compilers and their optimizations depend on a lot of things. Even the order and layout of instructions can matter due to the instruction cache. You can always go and make the guarantee later on, but undoing it would be impossible.

dathinab•1mo ago

> same uninitialized value, that can limit code generation options

it pretty much requires the compiler to initialize all values when they first "appear"

except that this is impossible and outright hazardous if pointers are involved

But doable for a small subset like e.g.

- stack values (but would inhibit optimizations, potentially pretty badly)

- some allocations e.g. I/O buffers, (except C alloc has no idea that you are allocating an I/O buffer)

dooglius•1mo ago

> If the compiler is required to make all code paths result in the same uninitialized value, that can limit code generation options

Can you provide (on say x86_64) an example of this, other than the case where the compiler prunes cases based on characterizing certain paths as UB? In other words, a case where "an uninitialized value is well-defined but can be different on each read" allows more performance optimization than "the value will be the same on each read".

> Also, an uninitialized value might be in a memory page that gets reclaimed and then mapped in again, in which case (because it hasn’t been written to) the OS doesn’t guarantee it will have the same value the second time. There was recently a bug discovered in one of the few algorithms that uses uninitialized values, because of this effect.

This does not sound correct to me, at least for Linux (assuming one isn't directly requesting such behavior with madvise or something). Do you have more information?

wrs•1mo ago

I can’t find a citation, but my recollection is the problem happened with the Briggs-Torczon sparse set algorithm, which relies on uninitialized memory not changing. For performance, they were using MMAP_UNINITIALIZED (which has to be enabled with a kernel config).

wrs•1mo ago

The most obvious general case (to me) is reading an uninitialized local variable in a loop. If uninitialized has to be the same value every time, you’d have to allocate a register or stack space to ensure the value was the same on every iteration. Instead, you’d don’t have to allocate anything, just use whatever value is in any register that’s handy. (By this logic you can also start pruning code, by picking the “most optimal” value for the uninitialized variable.)

vgatherps•1mo ago

Uninitialized memory being UB isn’t an insane default imo (although it makes masked simd hard), nor is most UB. But the lack of escape hatches can be frustrating

adastra22•1mo ago

Anything being UB is insane to me...

IshKebab•1mo ago

Nah it makes some sense for portability between architectures. Or at least it did back when C was invented and there were some wild architectures out there.

And it definitely does allow some optimisation. But probably nothing significant on modern out-of-order machines.

dathinab•1mo ago

> there were some wild architectures out there.

what is out there is still pretty wield

just slightly less

> probably nothing significant on modern out-of-order machines.

having no UB at all will kill a lot of optimizations still relevant today (and won't match anymore to hardware as some UB is on hardware level)

out of order machines aren't magically fixing that, just makes some less optimized code work better, but not all

and a lot of low energy/cheap hardware does have no or very very limited out of order capabilities so it's still very relevant and likely will stay very relevant for a very long time

dathinab•1mo ago

only until you get deeper into how the hardware actually work (and OS to some degree)

and realize sometimes the UB is even in the hardware registers

and that the same logical memory address might have 5 different values in hardware at the same time without you having a bug

and other fun like that

so the insanity is reality not the compiler

(through IMHO in C and especially C++ the insanity is how easily you might accidentally run into UB without doing any fancy trickery but just dumb not hot every day code)

uecker•1mo ago

I do not find it so easy to accidentally run into UB in C if you follow some basic rules. The exceptions are null pointer dereferences, out-of-bounds accesses for arrays, and signed overflow, all those can be turned into run-time traps. The rules include no pointer arithmetic, no type casts, and having some ownership strategy. None of those is difficult to implement and where exceptions are made, one should treat it carefully similar to using "unsafe" in Rust.

dooglius•1mo ago

There is no UB in hardware registers or physical DRAM, I don't think you actually have familiarity with how the hardware works if you make this claim. (Or perhaps you aren't familiar with how crazy "UB" in the sense of the ISO C documentation is)

EDIT: one could see "apparent" violation of memory consistency if say the cache subsystem or memory controller were misconfigured, however this would require both (1) you are running in kernel mode, not user-space (2) you have a bug, so GP's claim is not supported that bug-free code could encounter such a state.

addaon•1mo ago

> There is no UB in hardware registers or physical DRAM

This seems very sensitive to specific definitions that others might not share. DRAM is provided with a spec sheet that defines its behavior (if you write to an address, you’ll read back the same value from the same address in the future) under certain conditions. If you violate those conditions, the behavior is… undefined. If you operate DRAM with the wrong refresh timing, or temperature, or voltage, or ionizing radiation level, you may see strange behavior. Even non-local behavior, where the value read from one cell depends on other cells (RowHammer). How is this not UB?

dooglius•1mo ago

I've edited my claim to be a bit more clear, however in the context of parent's claim we are talking about bug-free code on an non-buggy physical processor, and I think implicitly we are talking about user-mode code where one does not have the ability to alter any DRAM timing configuration registers anyway.

adastra22•1mo ago

If a C program accesses uninitialized memory (UB), it is perfectly compliant with the language spec for the compiler to reformat your hard drive and replace your OS code with crypto mining.

I'm not exaggerating by a legalistic interpretation, and I'm only slightly exaggerating in practice. UB can do some really weird, unintuitive stuff in practice on real hardware:

https://mohitmv.github.io/blog/Shocking-Undefined-Behaviour-...

The point is that this extreme UB should never happen. It was a choice of the compiler implementors, and rather than fix this, they allowed the escape hatch of UB in the spec. It would be more sensible for the compiler to say that, e.g., accessing uninitialized memory results in a nonspecified value, or even possibly multiple different nonspecified values if accessed from different threads. That captures what we expect to happen, but would be (according to C language spec lawyers) defined behavior.

In practice, it would mean that compliant compilers will ensure that any situation in which uninitialized memory could be accessed would not result in weird edge case page faults on certain architectures or whatever that could in fact lead to wacky UB situations.

This is not an unreasonable ask.

addaon•1mo ago

> This is not an unreasonable ask.

But isn't this exactly parallel to the Rowhammer case in DRAM? When operating at the edge of the spec, the behavior of DRAM becomes undefined. (And, of course, one challenge with Rowhammer was about /which/ edge of the spec this happened on.) In this case, writing one physical address altered the contents of other physical addresses. This is "really weird, unintuitive stuff … on real hardware." And of course we can (and do) ask DRAM vendors to not take advantage of this undefined behavior; but they do so as an optimization, allowing slightly smaller and more closely spaced DRAM cells, and thus higher density DRAM dice for the same price. Just like it's possible to work with a language with fully-defined semantics at the cost of performance, it's possible to buy DRAM with much wider specifications and more clearly defined behavior up to the edges of those specifications… at the cost of performance.

Extreme UB in both hardware and software is a choice of priorities. You may favor giving up performance capabilities to achieve a more understandable system (so do I! I do most of my work on in-order lockstep processors with on-die ECC SRAM to maximize understandability of failure modes), but the market as a whole clearly does not, in both hardware and software.

adastra22•1mo ago

Legitimate bugs in hardware is probably out of scope for compilers. That would be an unreasonable ask, yes. But I believe it should be the job of the compiler to make sure that a correctly executed program on a reference machine strictly adhering to the architecture specs should not result in UB.

I'm not saying that a computer architecture should be UB-free. That would be awesome if it could be done, but in practice probably a bridge too far. But a compiler should map high-level directives into low-level implementations on a specific architecture using constructs that do not result in UB. This is not too much to ask.

A compiler can't reasonably protect you from rowhammer attacks. But it should guarantee that, barring hardware errors, accessing uninitialized memory has no effect other than something sensible like returning unspecified contents, or causing a memory access exception, or whatever. It should be defined up front what the behavior is, even if some of the runtime values are unpredictable.

As a more concrete example, most languages these days clearly define what happens in signed integer overflow: the thing that you expect to happen in any two's complement machine (char)127 + (char)1 == -128. C treats this as undefined behavior, and as mentioned in my link above that can cause what should be a finite loop (with or without overflow) to compile as an infinite loop. This "optimization" step by the compiler should never have happened. C resists changing this because C compilers exist for non-twos-complement architectures where the behavior would be different. IMHO the correct approach would be to require THOSE weird esoteric architectures to compile in extra overflow checks (possibly disabled with opt-in compiler flags that explicitly violate the standard), rather than burden every C developer everywhere with the mess that is signed arithmetic overflow UB.

It's a matter of expectations. Any but the most junior programmers expect that signed integer overflow will not be portable. That's fine. But they expect a sensible result (e.g. wrap around on two's complement machines), even if it is non-portable. They don't expect the compiler to silently and sneakily change the logic of their program to something fundamentally different, because UB means the compiler can do whatever tf it wants.

addaon•1mo ago

> Legitimate bugs in hardware is probably out of scope for compilers.

But that's exactly the point. The "bug" of RowHammer was that it occurred slightly on the "allowed" side of the envelope, at acceptably-low refresh rates. The "UB" of RowHammer and a hundred other observable effects is that, on the "disallowed" side of the envelope, the behavior is undefined. The system designer gets to choose at what probability they are on each side of the envelope, and the trade-offs are very much optimization opportunities.

Writing software in C that may exhibit undefined behavior is exactly this -- it's choosing, as a software engineer, to be on the far side of the specification envelope. In exchange, you get access to several powerful optimizations, some at the compiler level, and some at the career level (if you think that not needing to learn to use understand your language properly is at time optimization, at least).

Aurornis•1mo ago

> There is no UB in hardware registers

There most definitely is.

In the ARM documentation this is referred to as “UNPREDICTABLE”. The outcome is not defined. It may work. It may not. It may put garbage data in a register.

dooglius•1mo ago

I've edited/clarified the claim above. Parent did say "without you having a bug" and entering an UNPREDICTABLE state would be buggy code. Also, "maybe puts garbage data in a register" is not UB, it's a much more reasonable thing in UB (the definition of UNPREDICTABLE prior to ARMv8 does seem to allow for UB, however). I don't believe that such an UNPREDICTABLE state is reachable from user-space/unprivileged code (or else it would violate the chip's security properties) -- but if I'm wrong on that I'd be interested in an example.

adastra22•1mo ago

Here are just a few examples:

https://mohitmv.github.io/blog/Shocking-Undefined-Behaviour-...

dooglius•1mo ago

Those are examples regarding UB in the C++ language not the UNPREDICTABLE state in the ARM ISA which is what I asked for examples of.

imtringued•1mo ago

None of what you said makes any sense. You're mixing "UB" and bugs.

UB is about declaring programs invalid with a snide "don't do that", not about incorrect execution due to an incorrect specification. E.g. speculative execution running in privileged mode due to a prior syscall is just a plain hardware bug. It's not undefined behavior. In fact, the bug in question is extremely well defined.

The closest thing to reading undefined behavior is reading a "don't care" or VHDL's 'U' value from a std_ulogic and even those are well defined in simulation, just not in physical hardware, but even there they can only ever be as bad as reading a garbage value. Since a lot of the hardware design is non-programmable, there is also usually no way to exploit it.

jcranmer•1mo ago

How would you implement integer-to-pointer conversions without UB?

adastra22•1mo ago

What is UB about integer-to-pointer conversions?

jcranmer•1mo ago

Plenty of things! The resulting pointer may not be pointing to an object that is currently live, for example. It may not even be pointing to an object that makes any sense in the language's object model. It might be pointing to a return address on the stack, for example. Or a constant in the constant pool. Or a saved register in the middle of a computation that corresponds to no variables in the original program.

In short, the moment you enable integer-to-pointer conversions (assuming your target has a flat address space), you create pointer provenance problems whose only resolution is that some things have to be UB.

adastra22•1mo ago

I don't think any of those are undefined behavior in the strict sense in which the term is defined in the C/C++ standards. Pointer casts are defined behavior. I believe the things you point to are either implementation-defined or unspecified, which is different from UB.

It may seem nitpicky, but the downside of relying on implementation defined or unspecified behavior is largely boxed and contained. E.g you might get a memory access error. UB is, in principle, completely unlimited in downside. And because of that, it often interacts badly with optimization passes, resulting in very strange bugs.

pcwalton•1mo ago

jcranmer is correct and pointer provenance-related issues are not "boxed and contained". Start here: https://www.ralfj.de/blog/2020/12/14/provenance.html

jcranmer•1mo ago

Pointer provenance is UB in the C/C++ sense, although it's one that largely lurks in the implementation-defined behavior of "integer/pointer casts are implementation-defined." The closest you're going to find to a full specification of pointer provenance is TS 6010 (https://www.open-std.org/jtc1/sc22/WG14/www/docs/n3226.pdf), but it should be noted that no compilers actually implement provenance strictly along the lines of TS 6010. Instead, they implement a poorly-documented, buggy, internally incoherent definition of provenance...

> It may seem nitpicky, but the downside of relying on implementation defined or unspecified behavior is largely boxed and contained.

... that is incoherent precisely because the interactions of provenance with optimizations isn't boxed and contained.

It's really not until people started putting the model into formal semantics that they realized "hey, wait a second, this means either most of our optimizations are wrong or our semantics are wrong" and the consensus is that it was the semantics that were broken.

dathinab•1mo ago

> why primitive types aren't "frozen" by default.

it kills _a lot_ of optimizations leading to problematic perf. degredation

TL;DR: always freezing I/O buffers => yes no issues (in general); freezing all primitives => perf problem

(at lest in practice in theory many might still be possible but with a way higher analysis compute cost (like exponential higher) and potentially needing more high level information (so bad luck C)).

still for I/O buffers of primitive enough types `frozen` is basically always just fine (I also vaguely remember some discussion about some people more involved into rust core development to probably wanting to add some functionality like that, so it might still happen).

To illustrate why frozen I/O buffers are just fin: Some systems do already anyway always (zero or rand) initialize all their I/O buffers. And a lot of systems reuse I/O buffers, they init them once on startup and then just continuously re-use them. And some OS setups do (zero or rand) initialize all OS memory allocations (through that is for the OS granting more memory to your in process memory allocator, not for every lang specific alloc call, and it doesn't remove UB for stack or register values at all (nor for various stations related to heap values either)).

So doing much more "costly" things then just freezing them is pretty much normal for I/O buffers.

Through as mentioned, sometimes things are not frozen undefined on a hardware level (things like every read might return different values). It's a bit of a niche issue you probably won't run into wrt. I/O buffers and I'm not sure how common it is on modern hardware, but still a thing.

But freezing primitives which majorly affect control flows is both making some optimizations impossible and other much harder to compute/check/find, potentially to a point where it's not viable anymore.

This can involve (as in freezing can prevent) some forms of dead code elimination, some forms of inlining+unrolling+const propagation etc.. This is mostly (but not exclusively) for micro optimizations but micro optimizations which sum up and accumulate leading to (potentially but not always) major performance regressions. Frozen also has some subtle interactions with floats and their different NaN values (can be a problem especially wrt. signaling NaNs).

Through I'm wondering if a different C/C++ where arrays of primitives are always treated as frozen (and no signaling NaNs) would have worked just fine without any noticeable perf. drawback. And if so, if rust should adopt this...

Arnavion•1mo ago

>I’ve also dealt with pain implementing similar interfaces in Rust, and it really feels like you end up jumping through a ton of hoops (and in some of my cases, hurting performance) all to satisfy the abstract machine, at no benefit to programmer or application.

I've implemented what TFA calls the "double cursor" design for buffers at $dayjob, ie an underlying (ref-counted) [MaybeUninit<u8>] with two indices to track the filled, initialized and unfilled regions, plus API to split the buffer into two non-overlapping handles, etc. It certainly required wrangling with UnsafeCell in non-trivial ways to make miri happy, but it doesn't have any less performance than the equivalent C code that just dealt with uint8_t* would've had.

lilyball•1mo ago

What is the reason for explicitly tracking the initialized-but-unfilled portion? AIUI there's no harm in treating initialized bytes as uninitialized since you're working with u8, so what do you actually gain by knowing the initialized portion? I mean, at an API level you gain the ability to return a &mut [u8] for the initialized portion, but presumably anyone actually trying to write to this buffer either wants to copy in from a &[u8] or write to a &mut [MaybeUninit<u8>].

Arnavion•1mo ago

It's for the sake of providing a &mut [u8] to the initialized region for callers that cannot work with &mut [MaybeUninit<u8>] for whatever reason, eg if they wanted to use this with a std::io::Read impl. In particlar, Read impls are discouraged but not prohibited from reading the content of the [u8] given to them, so calling them with a [MaybeUninit<u8>] transmuted into a [u8] is not generally safe.

lilyball•1mo ago

Ah right, std::io::Read, I didn't think about that. And so it needs to explicitly track the initialized portion because otherwise calling .ensure_init() every time you want to do a read becomes expensive, that makes sense. And presumably this is also why the Buffer trait from this article cannot ever replace BorrowedBuf, because Buffer is not compatible with the Read trait without explicitly initializing the buffer every time (though I suppose it could still be added on top of BorrowedBuf if you add a new method init_len() that returns the length of the already-known-to-be-initialized prefix of parts_mut()).

EDIT: adding init_len() isn't good enough, we'd need an ensure_init() method too that returns a &mut [u8], and with those we could impl Buffer for BorrowedCursor, but if Read::read_buf() took this modified Buffer trait the default impl would be very inefficient when using a &mut [MaybeUninit<u8>] and that becomes a performance footgun.

jcranmer•1mo ago

This is already discussed for Rust: https://github.com/rust-lang/rfcs/pull/3605. TL;DR: it's not as easy as it looks to just add "freeze."

orlp•1mo ago

It is as easy as it looks to add `freeze`. That is, value-based `freeze`, reference-based `freeze` while seemingly reasonable is broken because of MADV_FREE.

Some people simply aren't comfortable with it.

Currently sound Rust code does not depend on the value of uninitialized memory whatsoever. Adding `freeze` means that it can. A vulnerability similar to heartbleed to expose secrets from free'd memory is impossible in sound Rust code without `freeze`, but theoretically possible with `freeze`.

Whether you consider this a realistic issue or not likely determines your stance on `freeze`. I personally don't think it's a big deal and have several algorithms which are fundamentally being slowed down by the lack of `freeze`, so I'd love it if we added it.

dathinab•1mo ago

also to be realistic in a lot of practical situations I/O buffers are reused so at least for the I/O buffer use-case it can be very viable (perf wise, in most use cases) to just zero or rand initialize it once on alloc and then treat it as frozen in all repeated usages (through it does open the issue of bug now potentially leaking previous content of the I/O buffer).

but I guess this isn't just about I/O buffers ;)

LegionMammal978•1mo ago

> Currently sound Rust code does not depend on the value of uninitialized memory whatsoever. Adding `freeze` means that it can.

Arguably, the existence of "asm!() freeze" has already broken this idea. Of course, you nominally don't get any guarantees about the stability of data that asm!() code reads from uninitialized bytes, yet you can do it nonetheless.

And it's not like it's practical to say "asm!() code is always unsound if it uses uninitialized bytes like they're numbers!", since lots of it does useful stuff like communicating with the kernel with structs that get serialized, and it can also open up interfaces like mmap() which translate possibly-uninitialized virtual-memory bytes into definite kernel bytes.

Not to mention /proc/self/mem and similar kernel-provided debugging utilities that can peek into memory as serialized data.

jcranmer•1mo ago

> Some people simply aren't comfortable with it.

Some people--especially when those people are closest to the workings of the operational semantics--not being comfortable is a sign that it is in fact harder than it looks.

The problems with "freeze" are in the same vein as integer-to-pointer semantics: it's a change which turns out to have implications for things not closely related to the operation itself, giving it a spooky-action-at-a-distance effect that is hard to tame.

The deeper issue is that, while there is clearly a use for some sort of "garbage value" semantics in a high-level language (that supports things like uninitialized I/O buffers, excessive reads for vectorization, padding bytes within structures), it's not clear which of the subtly different variants of garbage value works the best for all of the use cases.

JoshTriplett•1mo ago

This isn't just about the abstract machine. This is also about making it hard to end up using uninitialized memory, which is a security hole.

Abstractions like ReadBuf allow safe code to efficiently work with uninitialized buffers without risking exposure of random memory contents.

leoh•1mo ago

Just dropping to C for a smallish segment of a rust program kind of makes sense if you want to eke out performance here, no?

bjackman•1mo ago

You can always just use unsafe. This is about how to allow code to do this without unsafe blocks.

Arnavion•1mo ago

It does, though even for a small segment you end up needing a lot of boilerplate - a dependency on the cc crate, a build.rs that invokes the cc crate, extern declarations to tell the Rust caller about the C function, potentially some bindgen if your small segment is not so small ... and you still end up having to do some amount of thunking between & / * / MaybeUninit because of that anyway. So if there is a "pure Rust" way to do it with `unsafe`, writing that is often easier. The pure Rust impl also has the advantage that you can validate it with Miri, unlike the C impl case because Miri cannot emulate arbitrary C code.

leoh•1mo ago

This is a good point and perhaps points to an opportunity with cargo, macros, the rust language, or other aspects of the build system.

IshKebab•1mo ago

Yeah but the C toolchain is a huge pain, and makes things like cross-compiling and compiling to WASM harder. It's really nice if you can keep your program pure Rust.

Go has similar characteristics.

nottorp•1mo ago

> compiling to WASM

I thought Rust was supposed to be a systems language?

IshKebab•1mo ago

It is. And you can compile it to WASM.

treyd•1mo ago

Go is in a much worse situation here since the toolchain doesn't give you any help with C dependencies.

In Rust you can mostly just wrap the C library as a crate and the end developer doesn't need to care very much. If you want to use their system headers then you need to let them know, but that's not an issue if the headers are bundled. Cargo knows how to absorbs any C-built object files into the final binary and it just works.

In Go, in every scenario, the end developer needs to go through the effort of managing all the necessary the cgo calls when any of their dependencies use C libraries, the whole way up the dependency tree. It's really really not supported.

briansm•1mo ago

Pardon my ignorance, but I thought the whole point of Rust was to be a 'safe' modern alternative to C, so all new buffers would be zero'd at a neglible-these-days cost. Why is rust half-assing this?

vlovich123•1mo ago

It’s not. That is the case. But in cases where “negligible-these-days” isn’t quite negligible enough, this still matters and unsafe + MaybeUninit is the escape hatch to accomplish it.

Arnavion•1mo ago

Also not every type has a valid "all zeroes" value in the first place.

briansm•1mo ago

Yuck. In my mind, 'using C and not using Rust in the first place' is the escape hatch and Rust shouldn't even go there. Jeez, what a mess.

JoshTriplett•1mo ago

This is why we're spending substantial energy building better abstractions that don't require you to write any unsafe code.

vlovich123•1mo ago

Rust provides all the safety guarantees of managed languages with none of the performance drawbacks. It’s precisely intended to replace C/C++ because the unsafe parts of Rust are used very sparingly and result in significantly fewer bugs and security vulnerabilities.

Safe abstractions for dealing with uninitialized memory efficiently are important in very niche scenarios to get optimal code out of the compiler and for reducing the ability to make a mistake when writing such code.

Reaching for C to do this is an emotional overreaction instead of calmly dealing with a small corner case that already has workarounds even if it does involve using unsafe

cozzyd•1mo ago

what if your buffer is 64 GiB? (ok, in practice it will be zero'd on demand by the OS but still)

josephg•1mo ago

I’d love to know the actual performance impact of zeroing out memory. I bet the performance cost of zeroing out memory ahead of time is negligible in almost all programs.

cozzyd•1mo ago

Here is a really dumb example:

  $ cat bigbuf.c 

  #include <stdlib.h>
  #include <stdio.h>
  #include <string.h>

  #define DEFINITELY_BIG_ENOUGH 2U*1024*1024*1024
  int main(int nargs, char ** args)
  {

    char * definitely_big_enough = malloc(DEFINITELY_BIG_ENOUGH);

    if (nargs > 1)
    {
      memset(definitely_big_enough,0, DEFINITELY_BIG_ENOUGH);
    }

    sprintf(definitely_big_enough,"%s",args[0]);

    return 0;
  }
  
  $ /usr/bin/time ./bigbuf 
  0.00user 0.00system 0:00.00elapsed 100%CPU (0avgtext+0avgdata 1356maxresident)k
  0inputs+0outputs (0major+80minor)pagefaults 0swaps

  $ /usr/bin/time ./bigbuf 1 
  0.05user 1.25system 0:01.31elapsed 99%CPU (0avgtext+0avgdata 2098468maxresident)k
  0inputs+0outputs (0major+524369minor)pagefaults 0swaps

YMMV on different operating systems. Of course this is a program only an idiot would write, but things like caches are often significantly bigger than the median case, especially on Linux where you know there is overcommit.

Arnavion•1mo ago

A non-idiot would use calloc(DEFINITELY_BIG_ENOUGH), and that will likely erase the difference because the impl will be able to rely on mmap(ANONYMOUS) creating zero pages for such a large allocation. A more realistic test would be to have a large number of small allocations that get calloc'd and then free'd repeatedly, because a) free'ing a small allocation will not free the underlying page and thus reallocation will not be able to rely on it already being zero, and b) zeroing small allocations doesn't amortize the cost of zeroing as well as zeroing large allocations does.

delamon•1mo ago

c) SIMD won't be as helpful when you are zeroing small buffers of non-round sizes

Arnavion•1mo ago

That's what (b) is about.

briansm•1mo ago

Surely the point of Rust is 'safety at the price of performance' and if extra performance is required, don't use Rust. Don't bodge the language to accommodate!

int_19h•1mo ago

It is not. If it were, it wouldn't even have raw pointers, for example.

briansm•1mo ago

and yet an article about a 'safe' language has code examples full of 'unsafe'... wut.

panstromek•1mo ago

`unsafe` is just a barrier between what compiler proves and what programmer proves.

People should really read more on safety semantics in Rust before making comments like this, it's quite annoying to bump into surface level misunderstandings everytime Rust is mentioned somewhere.

hmry•1mo ago

In normal safe code, the compiler ensures your code has no UB. In an unsafe block, the programmer ensures the code has no UB.

Safe abstractions are created out of unsafe operations by making sure it's impossible to violate the operations' preconditions. Either by checking at runtime and returning an error or aborting the program if they're violated, or by using the type system and borrow checker to verify them at compile time (writing the code such that any program that could violate the preconditions must have a type error.)

If Rust didn't have unsafe, the only way to access the underlying unsafe operations would be dropping down to C/C++/Assembly, or hardcoding them in the compiler. This is what other languages do, and it's ergonomically worse because the barrier to adding a whole new language and build system to your project is quite high.

panstromek•1mo ago

The point of rust is explicitly not 'safety at the price of performance', quite the opposite. The whole point was to create language where safety doesn't cost performance like it does in most other languages.

arlort•1mo ago

The cost might not be negligible for everyone?

Rust is being used and is designed to be able to be used everywhere from top of the line PCs, to servers to microcontrollers to virtual machines in the browser.

Not all tradeoffs are acceptable to everyone all of the time

mrkeen•1mo ago

Other languages can easily achieve this kind of safety. What makes Rust different is that it tries to provide this level of safety without doing extra work at runtime (because otherwise people will put it in the same pile as Java/C#, and continue using C/C++ for speed).

sqrt_1•1mo ago

Related to unspecified vs undefined. I recall some C code was trying to be tricky and read from just allocated memory. Something like:

int* ptr = malloc(size); if(ptr[offset] == 0) { }

The code was assuming that the value in an allocated buffer did not change.

However, it was pointed out in review that it could change with these steps:

1) The malloc allocates from a new memory page. This page is often not mapped to a physical page until written to.

2) The reads just return the default (often 0 value) as the page is not mapped.

3) Another allocation is made that is written to the same page. This maps the page to physical memory which then changes the value of the original allocation.

Arnavion•1mo ago

A read from an unmapped page producing a different value than reading from that same page after it's mapped is an OS bug (*). If this was an already allocated page that had something written to it, reading from it would page it back in and then produce the actual content. If this was a new page and the OS contract was to provide zeroed pages, both the read before it was mapped and the read after it was mapped would produce zero.

What could happen is that the UB in that code could result in it being compiled in a way that makes the comparison non-deterministic.

(*): ... or alternatively, we're not talking about regular userspace program but a higher privilege layer that is doing direct unpaged access, but I assume that's not the case since you're talking about malloc.

sqrt_1•1mo ago

It was from C++con 2016 - Facebook take on small strings https://www.youtube.com/watch?v=kPR8h4-qZdk&t=1343s I believe it is about a page that was conditionally returned to the kernel.

Arnavion•1mo ago

The speaker was mistaken / misspoke.

The closest thing to "conditionally returned to the kernel" is if the page had been given to madvise(MADV_FREE), but that would still not have the behavior they're talking about. Reading and writing would still produce the same content, either the original page content because the kernel hasn't released the page yet, or zero because the kernel has already released the page. Even if the order of operations is read -> kernel frees -> write, then that still doesn't match their story, because the read will produce the original page content, not zero.

That said, the code they're talking about is different from yours in that their code is specifically doing an out-of-bounds read. (They said "If you happen to allocate a string that's 128 bytes, and malloc happens to return an address to you that's 128 bytes away from the end of the page, you'll write the 128 bytes and the null terminator will be the first byte on the next page. So they're very clearly talking about the \0 being outside the allocation.)

So it is absolutely possible to have this setup: the string's allocation happens to be followed by a different allocation that is currently 0 -> the `data[size()] != '\0'` check is performed and succeeds -> `data` is returned to the caller -> whoever owns that following allocation writes a non-zero value to the first byte -> whoever called `c_str()` will now run off the end of the 128B string. This doesn't have anything to do with pages; it can happen within the bounds of a single page. It is also such an obvious out-of-bounds bug that it boggles my mind that it passed any sort of code review and required some sort of graybeard to point out.

sqrt_1•1mo ago

I don't believe they are allocating 128 bytes, or accessing out of bounds memory.

He explicitly states 128byte filename allocates 129 bytes. https://www.youtube.com/watch?v=kPR8h4-qZdk&t=1417s

Arnavion•1mo ago

In that case the bug he described simply does not exist.

sqrt_1•1mo ago

This is well outside my expertise, but some discussion happened at the time https://www.reddit.com/r/programming/comments/56xxmb/the_str...

Some people suggest that maybe Facebook runs with MAP_UNINITIALIZED

Arnavion•1mo ago

Unfortunately this hypothesis is also wrong. MAP_UNINITIALIZED can only be enabled in the kernel when there is no MMU, and in that case the page will already be in physical memory, so the very first pointer dereference will read the correct byte, not a fake zero because it's "uninitialized".

mleonhard•1mo ago

> without doing anything hugely inefficient, such as initializing the full buffer

Is this so inefficient? If your code is very sensitive to IO throughput, then it seems preferable to re-use buffers and pay the initialization once at startup.

Some years ago, I needed a buffer like this and one didn't exist, so I wrote one: https://crates.io/crates/fixed-buffer . I like that it's a plain struct with no type parameters.

MaulingMonkey•1mo ago

> Is this so inefficient?

It can be. If you have large buffers (tuned for throughput) that end up fulfilling lots of small requests for whatever reason, for example. And there's always the occasional article when someone rediscovers that replacing malloc + memset with calloc can have massive performance savings thanks to zeroing by the OS only occuring on first page fault (if it ever occurs), instead of an O(N) operation on the whole buffer up front.

Which, if in the wrong loop, can quickly balloon from O(N) to O(scary).

https://github.com/PSeitz/lz4_flex/issues/147

https://github.com/rust-lang/rust/issues/117545

If I'm reading that log-log plot right, that looks like a significantly worse than 100x slowdown on 1GB data sets. Avoiding init isn't the only solution, of course, but it was a solution.

> then it seems preferable to re-use buffers

Buffer reuse may be an option, but in code with complicated buffer ownership (e.g. transfering between threads, with the thread of origination not necessarily sticking around, etc.), one of the sanest methods of re-use may be to return said buffer to the allocator, or even OS.

> and pay the initialization once at startup.

Possibly a great option for long lived processes, possibly a terrible one for something you spawn via xargs.

pornel•1mo ago

Couple of things that are commonly misunderstood/unappreciated about this:

• Uninitialized bytes are not just some garbage random values, they're a safety risk. Heartbleed merely exposed unitialized buffers. Uninit buffers can contain secrets, keys, and pointers that help defeat ASLR and other mitigations. As usual, Rust sets the bar higher than "just be careful not to have this bug", and therefore the safe Rust subset requires making uninit impossible to read.

• Rust-the-language can already use uninitialized buffers efficiently. The main issue here is that the Rust standard library doesn't have APIs for I/O using custom uninitialized buffers (only for the built-in Vec, in a limited way). These are just musings how to design APIs for custom buffers to make them the most useful, ergonomic, and interoperable. It's a debate, because it could be done in several ways, with or without additions to the language.

ajross•1mo ago

> Uninitialized bytes are not just some garbage random values, they're a safety risk.

Only when read. Writing to "uninitialized" memory[1] and reading it back is provably secure[2], but doesn't work in safe Rust as it stands. The linked article is a proposal to address that via some extra complexity that I guess sounds worth it.

[1] e.g. using it as the target of a read() syscall

[2] Because it's obviously isomorphic to "initialization"

pornel•1mo ago

Obviously, initialized memory isn't an uninitialized memory any more.

There are fun edge cases here. Writing to memory through `&mut T` makes it initialized for T, but its padding bytes become de-initialized (that's because the write can be a memcpy that also copies the padding bytes from a source that never initialized them).

lilyball•1mo ago

Note that if you have a `&mut T` then the memory must already be initialized for T, so writing to that pointer doesn't initialize anything new (although as you say it can deinitialize bytes, but that only matters if you use transmute or pointer casting to get access to those padding bytes somehow).

az09mugen•1mo ago

With so much unsafe, it makes me think of that fun experimental usage of rust : https://github.com/tsoding/Crust

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Data on AI-related Show HN posts

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Get the location of the ISS using DNS

Functions Are Vectors (2023)

I don't think AGI is right around the corner

There's a COMPUTER inside my DS flashcart [video]

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Overclocking LLM Reasoning: Monitoring and Controlling LLM Thinking Path Lengths

Paper Shaders: Zero-dependency canvas shaders

Writing into Uninitialized Buffers in Rust

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